Lubricating oils as used in, for example, the internal combustion engines of automobiles or trucks are subjected to a demanding environment during use. This environment results in the oxidation of the oil catalyzed by the presence of impurities in the oil, such as iron compounds, and is also promoted by the elevated temperatures experienced by the oil during use. This catalyzed oxidation of the oil not only contributes to the formation of corrosive oxidation products and sludge in the oil but can also cause the viscosity of the oil to increase or even cause the oil to solidify. This oxidation of lubricating oils during use is usually controlled to some extent by the use of antioxidant additives which may extend the useful life of the oil, for example, by reducing or preventing unacceptable viscosity increases.
Among the additives that are known to be effective in extending the life of lubricating oils are phenolic antioxidants and aminic antioxidants. Phenolic antioxidants contain one or more sterically hindered phenol functionalities, and can be either used alone or in synergistic combination with other additives, such as alkylated aminic antioxidants. Aminic antioxidants contain one or more nitrogen atoms and typically comprise alkylated diphenyl amines and phenothiazines. The synthesis and uses of phenolic antioxidants, phenothiazines and alkylated diphenyl amines have been reported in the literature.
For example, it is known from U.S. Pat. No. 4,824,601 to produce a liquid antioxidant composition by reaction of diphenylamine with diisobutylene in a molar ratio from 1:1.1 to 1:2.5 in the presence of an acid-activated earth catalyst, at a reaction temperature of at least 160° C. The reaction product is a mixture of t-butylated diphenylamines; t-octylated diphenylamines; and (iii) higher alkylated diphenylamines, in which the content of 4,4′-di-t-octyldiphenyl-amine in the final reaction mass, excluding catalyst, is below 25% by weight and that of diphenylamine is below 10% by weight. The product is said to be an excellent anti-oxidant for lubricants and functional fluids of mineral oil and synthetic origin.
In addition, U.S. Patent Publication No. 2005/0230664 discloses a lubricant anti-oxidant mixture prepared by the partial condensation of an alkylated diphenylamine with an aldehyde or ketone in the presence of an acidic catalyst to yield at least one acridan of the general formula:
wherein: R1, R2, R3, and R4 are independently selected from the group consisting of hydrogen, C3 to C32 alkyl, and C3 to C32 alkenyl, provided that at least one of R1, R2, R3, and R4 is not hydrogen, and R5 and R6 are independently selected from the group consisting of C1 to C20 hydrocarbyl and hydrogen; wherein, at the termination of said condensation, residual alkylated diphenylamine is not separated from the acridan product.
In addition, of the many commercially available amine anti-oxidants Naugalube® 403 is supplied by Chemtura Corporation and is N,N′-di-sec-butyl-p-phenylene diamine of the following formula:

There is, however, a continuing need for new antioxidants and antioxidant systems which offer improved performance and which are effective at low levels. There are a number of factors which have contributed to this continuing need. One such factor is that in recent years internal combustion engines are often operated at even higher temperatures, which tend to increase the rate of oxidation and shorten the useful life of the oil. In addition, there is a strong desire to use cheaper base stocks for lubricating oil compositions which have inferior resistance to oxidation and require more efficient and effective antioxidants. There is also a need for lubricating oils to have a longer in service life span to support the longer service intervals for motor vehicles.
According to the present invention, a novel class of dialkylanilino-cyclohexane compounds has now been prepared, which compounds have been found to exhibit advantageous anti-oxidant properties when used alone or with other additives in lubricant formulations, fuel formulations and rubber compositions.
1,4-Bis(phenylamino)cyclohexane is a known compound and, according to Russian Patent No. 755,812, is useful in combination with hydroquinone in increasing the scorch resistance of synthetic rubbers.